Phycobilisomes Harbor FNR_L in Cyanobacteria

Area of Science:

• Photosynthesis
• Biochemistry
• Structural Biology

Background:

• Cyanobacterial phycobilisomes (PBSs) are crucial light-harvesting antenna complexes.
• PBSs supply energy to reaction centers for photochemistry.
• Two main PBS types exist: CpcG-PBS and CpcL-PBS, with CpcL-PBS composition being unclear.

Purpose of the Study:

• To isolate and characterize the protein composition of CpcL-PBS from *Synechocystis* sp. strain 6803.
• To elucidate the functional association of ferredoxin-NADP+ oxidoreductase (FNR_L) with PBSs.
• To understand the structural basis of FNR_L-PBS interaction.

Main Methods:

• Isolation and characterization of CpcL-PBS.
• Fluorescence spectroscopy (room temperature and low-temperature).
• SDS-PAGE and quantitative mass spectrometry.
• Isotope-encoded cross-linking mass spectrometry (IXMS) combined with computational protein structure prediction and modeling.

Main Results:

• CpcL-PBS was isolated and characterized, revealing it lacks allophycocyanin (APC).
• Ferredoxin-NADP+ oxidoreductase (FNR_L) is tightly associated with both CpcL-PBS and CpcG-PBS.
• CpcL-PBS exhibits red-shifted emission (669 nm) and increased FNR_L and CpcC2 content, suggesting longer rod lengths.
• A molecular model of FNR_L-PBS binding was established, supported by cross-linking data.

Conclusions:

• A novel functional assembly of phycobiliproteins (CpcL-PBS) was identified.
• The close association of FNR_L with PBSs provides new insights into regulating light energy transfer and carbon fixation.
• FNR_L's role in fine-tuning energy flow via cyclic and noncyclic electron transport is highlighted.